1. Field of the Invention
This invention relates to an optical apparatus having an automatic focusing device arranged to set a movement inhibiting area within a moving range of a focusing lens.
2. Description of Related Art
It was generally practiced in the past to manually bring a lens into focus on an object to be photographed in taking a shot with a camera. During recent years, however, most of cameras or lenses have come to be arranged to automatically make focus adjustment by driving focusing lenses with actuators such as DC and ultrasonic motors or the like mounted in the cameras or lenses. The automatic focus adjustment (automatic focusing) enhances focusing accuracy and shortens to a great extent a length of time required in focusing to enable everyone to easily take a sharp picture without missing shutter operating opportunities.
However, the time required for focusing varies depending on various conditions and parameters. For example, a relatively long time is required in moving a focusing lens when the moving area of the focusing lens is wide depending on the focal length of the lens. Further, the focusing time is not always short depending on parameters such as luminance of the photographing object obtained by a distance measuring sensor, a perspective relation of a main object to peripheral objects, the focusing sensitivity of the lens, the relation of the current lens position to that of the photographing object, etc. Further, the lens might be focused on a wrong object when objects located at different distances appear together in the same field at far and near distances in a commingling state. This state is called "a far-and-near-distance competing state".
To solve this problem, in U.S. Pat. No. 4,284,335, there is disclosed a camera, which is arranged to permit selection of any object as desired, to set a distance measurement inhibiting zone within the whole distance measuring area of a distance measuring sensor and to inhibit any focusing action within the distance measurement inhibiting zone.
In another prior art of lenses, there is a lens system arranged to divide a moving area of a focusing lens between a nearest distance end position and an infinity distance end position into two areas, i.e., a movement allowing area and a movement inhibiting area, and to limit a moving range of the focusing lens by judging the moving direction indicated by an instruction coming from the camera to move the focusing lens. This enables the lens to quickly carry out automatic focusing control.
FIG. 6 is a diagram for explaining the conventional method for limiting a moving range of a focusing lens.
Referring to FIG. 6, the whole area of a moving range of the focusing lens is an area between a nearest distance end position and an infinity distance end position. The lens of the automatic focusing type is provided with a position detecting means for detecting the absolute position of the focusing lens and a distance limit switch for designating a moving area.
When the distance limit switch is in its off-state, the movable range of the focusing lens becomes the whole area (an A zone and a B zone). When the distance limit switch is in its on-state, the movable range of the focusing lens becomes the B zone only.
In a case where the distance limit switch changes from an off-state to an on-state while the absolute position of the focusing lens is in the B zone, the focusing lens becomes movable within the B zone only. In other words, in this case, the focusing lens is inhibited from entering the A zone. In this case, a photographing object desired is naturally located within the B zone in general and the lens is effectively prevented from being focused on a wrong object.
Further, if the distance limit switch changes from an off-state over to an on-state while the focusing lens is in the A zone, the focusing lens can be moved by either of the following two methods. According to a first method, while the focusing lens is in the A zone, the focusing lens can not be driven by an automatic focusing action and must be moved to the B zone by a manual operation by the operator. According to a second method, while the focusing lens is in the A zone, the distance limit switch is considered to be in its off-state, even if the distance limit switch is actually in its on-state. Then, the focusing lens is driven to move to the B zone and, once the focusing lens enters the B zone, the distance limit switch is completely turned on to make it impossible for the focusing lens to return to the A zone.
However, in accordance with the first method, shutter operating opportunities might be missed, because the manual operation required cannot be carried out fast enough. In accordance with the second method, if the focusing action happens to encounter any focusable object other than a desired object during the process of moving the focusing lens to the B zone, the focusing action might be performed on the wrong object. As a result, the time required in bringing the focusing lens to the B zone becomes longer than a length of time normally required.